The present invention relates to route selection control for a packet switching network, and more particularly to a packet switching network having a function to repeat avoiding a looped trunk caused by network failure or the like.
As a system for repeating a packet in a packet switching network, there are known a fixed alternate trunking system and an adaptive alternate trunking system. According to the former system, a route to a destination of a packet is determined beforehand at each repeater node. According to the latter system, a route to a destination is determined in accordance with network traffics and failure conditions. If a failure occurs in the fixed alternate trunking system, the repeater nodes connected to the failure site change their trunking route in accordance with a predetermined rule. Therefore, if the scale of a packet switching network is large, there may occur a looped trunk in the network along which a packet is continuously transmitted. In such a case, the packet once entered into the looped trunk cannot escape therefrom to thereby disable packet communication.
As a measure for dealing with a looped trunk generated in a packet switching network, there have been known, as described in, e.g., "Computer Network" edited and written by Yagi et al., published by Asakura Shoten on June 15, 1983, at page 24, a method of discarding a packet if the number of repeating times of the packet exceeds a predetermined number, and a method of discarding a packet if it stays within the network longer than a predefined time.
The above-described prior art does not pay regard to repeating the packets, which enter into a looped trunk, avoiding the looped trunk after the looped trunk was brought about. The succeeding packets are discarded under the condition that the number of repeating times or the stay time exceeds a certain value, posing a problem of disability of packet communication.